Methanol is produced commercially from hydrogen-rich synthesis gas in a packed bed catalytic reactor operated in the gas phase with means for removing heat from the highly exothermic methanol synthesis reaction. Synthesis gas is shifted when necessary so that the reactor feed is hydrogen-rich and dilute in carbon oxides, typically containing carbon monoxide concentrations no greater than 6 to 9 vol%. Hydrogen-rich unreacted synthesis gas is recycled back to the reactor in order to moderate the reactor temperatures and increase the overall conversion to methanol. Unreacted synthesis gas is enriched in hydrogen prior to recycle in certain cases when more unreacted synthesis gas is produced than is needed for the plant fuel system.
Methanol synthesis alternately can be accomplished in a liquid phase reactor system in which synthesis gas is reacted in the presence of a powdered catalyst suspended in an inert liquid, which allows much higher per pass conversion and more effective removal of reaction heat and control of catalyst temperature compared with a gas-phase reactor system. The liquid phase methanol process is described in detail in U.S. Pat. No. 4,031,123 and 4,567,204, the specifications of which are incorporated herein by reference.
The liquid phase methanol (LPMEOH) process can be operated in different modes depending on specific applications. An article by G. W. Roberts et al entitled "The Liquid Phase Methanol Process--an Efficient Route to Methanol from Coal" presented at the Conference on Coal Gasification and Synthetic Fuels for Power Generation, San Francisco, 14-18 April 1985, describes the operation of the LPMEOH process on a once-through basis using unshifted, CO-rich synthesis gas in which the process is integrated with a coal gasifier in a coal gasification combined cycle (CGCC) power generation system. Unconverted synthesis gas is utilized as gas turbine fuel and methanol is stored for use as gas turbine fuel during peak power demand. The LPMEOH process also can be operated in a standalone mode to maximize methanol production. In this case, if CO-rich synthesis gas feed is utilized, the gas must be shifted prior to the reactor to yield the required 2:1 stoichiometric H.sub.2 /CO molar ratio for methanol synthesis, and unconverted gas is recycled directly to the reactor to increase overall methanol yield. Further descriptions of once-through LPMEOH systems are given by Studer et al in a paper entitled "Status Report on the Liquid Phase Methanol Project" published in the Proceedings of the 14th Annual EPRI Conference on Fuel Science, GS-6827, May, 1990, p. 16-1 et seq. and in a report by Chem Systems, Inc. entitled "Optimization of Electricity-Methanol Production", Final Report, GS-6869, prepared for the Electric Power Research Institute, June, 1990.
The operation of a LPMEOH reactor system with recycle of unreacted gas to achieve high conversion requires a stoichiometric feed, as pointed out in a report by the Bechtel Group entitled "Slurry Reactor Design Studies: Slurry vs Fixed-Bed Reactors for Fischer-Tropsch and Methanol", Final Report to the U. S. Department of Energy, DOE/PC/89867-T2, June 1990 at p. 24; the unreacted synthesis gas is recycled directly to the reactor as shown in FIG. E-1 at p. 188. This requirement for stoichiometric or shifted feed gas for the operation of LPMEOH reactors in the recycle mode is also described in a report by A. B. Walters and S. S. Tam entitled "Methanol Coproduction with a Baseload IGCC Plant", 9th EPRI Conference on Coal Gasification Power Plants, Palo Alto, Calif., October 1990, at pp. 3-4 and Slide #8. Further discussion of this requirement is given by M. Sherwin and D. Blum in their report entitled "Liquid Phase Methanol", AF-1291, Final Report to the Electric Power Research Institute, December 1979, at p. 3-7. Koppers-Totzek synthesis gas is CO-rich as shown in Table 3--3 at p. 3-5.
An alternate method for the operation of a LPMEOH reactor with recycle is described in U.S. Pat. No. 4,946,477 in which the reactor is fed with CO-rich synthesis gas, the unreacted synthesis gas is separated into a hydrogen-rich stream and a CO-rich stream, and the hydrogen-rich stream is recycled to the reactor feed (FIG. 7). Alternately, water can be added to the synthesis gas feed to promote CO shift within the LPMEOH reactor (FIG. 8). Addition of water to a CO-rich synthesis gas feed is also discussed by M. Sherwin and D. Blum in a report entitled "Liquid Phase Methanol", AF-693, Interim Report to the Electric Power Research Institute, May 1978, at p. 2--2. It is stated that a hydrogen-rich unconverted reactor effluent gas can be obtained by water addition to a CO-rich synthesis gas feed, and that water co-feeding may be of interest where total conversion to methanol is required and the reactor effluent gas is recycled.
U.S. Pat. No. 4,766,154 discloses a two-stage LPMEOH reactor system operated in series using hydrogen-rich synthesis gas feed. Unreacted synthesis gas from the first stage passes to the second stage reactor, and unreacted synthesis gas from the second stage reactor is recycled to the second stage reactor feed. The synthesis gas feed, unreacted synthesis gas from the first stage reactor, and unreacted synthesis gas from the second stage reactor are all hydrogen-rich streams as given in Table I.
The operation of a LPMEOH reactor using stoichiometric or hydrogen-rich synthesis gas feed yields a stoichiometric or hydrogen-rich reactor effluent stream as is known in the art. When such a LPMEOH reactor is operated in the recycle mode, the recycle stream is thus hydrogen-rich. As noted above, a LPMEOH reactor operating with a CO-rich synthesis gas feed with water addition can produce a hydrogen-rich effluent gas, which is favored for recycle operation when total syngas conversion to methanol is desired. The operation of LPMEOH reactors in the recycle mode as disclosed in the prior art described above thus involves exclusively the recycle of stoichiometric or hydrogen-rich streams to the reactor feed.